Method and apparatus for dielectric heating



P 2, 1969 w. J. BLEACKLEY ET AL 3,465,114

METHOD AND APPARATUS FOR DIELECTRIC HEATING Filed Sept. 19. 1966 2 Sheets-Sheet 1 p 2, 1969 -w. J. BLEACKLEY ETAL 3,465,114

METHOD AND APPARATUS FOR DIELECTRIC HEATING Filed Sept. 19. 1966 2 Sheets-Sheet 2 United States Patent 3,465,114 METHOD AND APPARATUS FOR DIELECTRIC HEATING William J. Bleackley and William A. Cumming, Ottawa,

Ontario, Canada, assignors to Canadian Patents and Development Limited, Ottawa, Ontario, Canada, a company of Canada Filed Sept. 19, 1966, Ser. No. 580,428 Int. Cl. Hb 9/06 US. Cl. 219-1055 4 Claims ABSTRACT OF THE DISCLOSURE Microwave energy is propagated along a circular waveguide in the TE mode to yield a concentration of lines of electric force in the portion of the waveguide occupied by a hollow cylindrical workpiece to be heated. In an alternative the microwave energy is propagated as a circurlarly polarised wave for heating a similar workpiece.

This invention relates to improvements in methods and apparatus for dielectric heating, that is to say the heating of materials by microwave energy. In one particular application, the invention is concerned with the drying of elongated articles by heating the water entrained therein by the application of microwave energy.

In copending United States patent application Ser. No. 563,606 of William A. Cumming filed July 7, 1966 there are described methods and apparatus for the dielectric heating of elongated workpieces in the form of webs or filaments. Such copending application describes a method in which the workpiece is caused to travel along its own longitudinal axis along the interior of a waveguide, while microwave energy is propagated along the waveguide in either direction along said axis. The energy is chosen to have a wavelength at which at least a part of the workpiece will absorb energy to generate heat. In the most common application of the invention, namely that of drying, it will be water entrained in the workpiece that absorbs the energy and is consequently evaporated. In such prior application, the manner of propagation of the microwave energy along the Waveguide is chosen to be such as to include an operating mode that yields a relatively high concentration of lines of electric force in the portion of the waveguide physically occupied by the workpiece.

The present invention is a further development of the invention disclosed in such other application.

In one aspect the present invention is particularly di rected towards the heating of hollow cylindrical workpieces. One example of such a workpiece is a plastic tube or skin of the type employed in modern sausage making machines to contain the meat during the cooking operation. It is often necessary as part of the manufacture of such tubes that they be heated before use to drive olf excess moisture. It is convenient that this heating should be carried out by microwave energy, which method tends to have the advantage of avoiding excessive heating of the material itself, while rapidly evaporating the water entrained therein.

In another aspect the present invention is directed towards a new method of heating solid cylindrical workpieces, more especially those having a substantial diameter.

The object of the present invention is thus to provide modifications of the methods and apparatus disclosed in said other application, such modifications being especially directed towards the heating of cylindrical workpieces. Specifically, it is an object of the present invention to provide methods and apparatus for enhancing the energy transfer between the microwave energy and the cylindrical workpiece, and for obtaining improved uniformity of heating throughout the workpiece.

Various manners in which the invention may be carried into practice are illustrated in the accompanying drawings. It is to be understood that the specific forms of the invention illustrated in the drawings are provided by Way of example only and not by way of limitation of the invention, the broad scope of which is defined in the appended claims.

In the drawings:

FIGURE 1 shows an end view of a circular waveguide illustrating the lines of electric force in one mode of operation; I

FIGURE 2 is a partly cut away view (taken generally on the line IIII in FIGURE 3) of a waveguide structure for use in carrying out the present invention;

FIGURE 3 is a section on the line III--III in FIGURE 2;

FIGURE 4 is a fragmentary view showing a modified form of the right hand end of FIGURE 3;

FIGURE 5 is a fragmentary view showing a modified portion of FIGURE 2;

FIGURE 6 shows yet a further modification of FIGURE 5;

FIGURE 7 is an end view of a circular waveguide illustrating a different mode of operation;

FIGURE 8 is an end view of an alternative construction; and

FIGURE 9 shows diagrammatically a method of feeding a hollow cylindrical workpiece along a waveguide.

FIGURE 1 shows a circular wave-guide 10, the arrows 11 illustrating the lines of electric force that will exist in this waveguide when the same is energized to operate in the transverse electric mode known as the TE mode (using the usual United States nomenclature). It will be noted that the lines of electric force 11 are circular and that they are concentrated in the area 12. The lines of magnetic force have not been illustrated in FIGURE 1, since they are of no interest to the present invention, the heating energy transfer being effected solely by the electric field. In accordance with the present invention, this feature of the TE mode of operation in a circular waveguide is exploited by causing a tubular cylindrical workpiece 13 (FIGURES 2 and 3) to travel along the Waveguide 10 with its wall in the high concentration region 12. Since the location of the maximum concentration region 12 is normally equidistant between the wall of the waveguide 10 and the central longitudinal axis thereof, the positioning of the workpiece 13 at this 1ocation will be facilitated by making the diameter of the waveguide 10 approximately twice that of the workpiece 13. In this Way the workpiece 13 is caused to lie in the cylindrical location at which a maximum number of lines of electric force exist. In reality, the presence of the workpiece 13 will have the effect of further concentrating the field, so that the concentration in the region 12 is in fact far greater than can be illustrated in FIGURE 1.

In a circular waveguide, the dominant mode will be the TE mode. After this will come the TM and TE modes, before the modes TM and TE which will appear simultaneously at the same wavelength. It is thus necessary to choose the frequency of the microwave energy in relation to the dimensions of the waveguide, such that all these five modes can theoretically appear, and then so to stimulate the waveguide that only the TE mode is propagated, the other four modes mentioned above being suppressed.

Subject to possible variation to accommodate the foregoing, the preferred range of frequencies for the microwave energy is from 20 cm. to- 1 cm. Microwave energy in the region of 2 cm. is the most effective for drying purposes, because the peak in the loss curve for water occurs around 2 cm. On the other hand, energy sources are more readily available and are less expensive at lower frequencies, which considerations will usually encourage the use of the longer wavelengths. Another and sometimes controlling factor is the diameter of the waveguide as dictated by the diameter of the workpiece.

One method of injecting microwave energy into a circular waveguide in the TE mode with suppression of the other four modes mentioned above is illustrated in FIGURES 2 and 3. These figures show the injection of microwave energy centrally into the waveguide 10, to flow simultaneously in opposite directions from the central area 14 to the two ends 15 of the waveguide. At the ends any residual energy is absorbed by cylindrical attenuating wedges 16. The microwave energy from a source 17 is fed first into a rectangular waveguide 18, which is bifurcated to form further rectangular waveguides 18m, which in turn are bifurcated to form waveguide arms 19 from which the energy is injected into the main waveguide 10 through four slots 20 located circumferentially around the Waveguide 10 at 90 to each other. It has been discovered that, with the source 17 energising the rectangular waveguide 18 in the TE mode, this particular method of ejecting the energy into :a circular waveguide yields stimulation therein of the TE mode, to the exclusion of the equal or more dominant modes TE TE TM and TM Provided the waveguide is smoothly electrically continuous and no irregularities exist on which the other modes can form, the propagation will continue substantially exclusively in the TE mode shown in FIGURE 1.

The workpiece 13 is caused to travel along the waveguide 10 in either direction. The direction chosen is unimportant, since, whichever way it travels, it will be travelling in the same direction as the energy propagation at one end of the waveguide and against the direction of energy propagation at the other end of the waveguide, and will always enter the waveguide in the latter condition, namely in opposition to the direction of energy flow.

In one form of the invention the waveguide is terminated by means of the attenuating wedges 16. This avoids reflection of the microwave energy and the setting up of standing waves. In other words the energy is propagated in a single travelling wave, in contrast to the double travelling wave that is the equivalent of a standing wave.

Alternatively, the waveguide 10 may be terminated at both ends by a metal reflector, as shown at 40 in FIGURE 4. Beyond the reflector 40, is a short waveguide 41, which will be arranged to be of too small a diameter to propagate energy at the wavelength being used. Under these circumstances the system can be caused to act as a resonant cavity, with the aid of a conventional tuner 42 provided in the waveguide 18 (FIGURE 5), or with the aid of conventional tuner 42, cross guide directional coupler 43, detector 44 and meter 45 (FIGURE 6). By adjusting the tuner 42 until the meter 45 shows minimum reflected energy, maximum energy transfer to the workpiece 13 can be achieved.

It will normally be unnecessary to provide any supports in the waveguide itself for the workpiece, assuming the workpiece to be a relatively light material such as a cylindrical sausage skin. If the workpiece does require support in the waveguide, then this can readily be supplied by suitable structure made from a material transparent to the microwave energy.

It is already standard practice to pass sausage skins through conventional thermal heaters. The skin is extruded from a nozzle 30 of a forming machine 31 (FIG- URE 9) which nozzle includes a central perforated die through which air is injected into the cylindrical tube of skin 13 as it is formed. This air serves to keep the workpiece straight and comparatively rigid and hence selfsupporting between spaced sets of rollers 32 and 33 and particularly self-supporting in the span extending along the waveguide 10. This flow of air also serves to remove water vapour generated inside the workpiece during the drying process. Air may also be blown down the waveguide on the outside of the workpiece for the same purpose. In practice, manufacturing considerations will require other operations to be carried out on the skin 13 between the nozzle 30 and the drying process that is carried out in the waveguide 10. These are signified by the break in the workpiece 13 and are not further described, since they are not pertinent to the present invention.

As an alternative to use of the TE mode, energy propagated with circular polarization may be employed, as is demonstrated in FIGURE 7. This view shows a circular waveguide 21 excited by a pair of orthogonal probes 22, 23 which will be energised in time quadrature. The solid arrows 24 represent the instantaneous electric field when energised in the T E mode. The broken arrows 25 represent the same field later. The effect is thus of a rotating field of constant amplitude. This method of energising the waveguide ensures uniformity of heating around the circumference of the workpiece (not shown in FIGURE 7 to simplify illustration, but in practice extending coaxially along the waveguide, as in the previous embodiments). This manner of operation is not only applicable to hollow cylindrical workpieces; it is also applicable to solid filamental workpieces, more particuary to those having a substantial diameter.

All the manners of operation described herein and especially the latter manner of operation (i.e. with a circularly polarized field) are not only applicable to circular waveguides, but to all shapes of waveguide, more especially to those having orthogonal symmetry, that is to say to those having a shape that can be turned through 90 about the longitudinal axis while remaining basically the same in appearance, e.g. a circle or a square. FIGURE 8 shows another such form of waveguide having orthogonal symmetry that may be used in conjunction with energy propagated with circular polarization. This is a ridged square waveguide 26 energised, as in the case of the waveguide 21, by a pair of orthogonal probes 27, 28 excited in time quadrature. Along the centre of this waveguide a tubular or solid workpiece 13 is caused to travel in basically the same manner as in FIGURES 2 and 3. This particular shape of waveguide, when excited in the TE mode in both directions in quadrature provides circular polarization of the lines of electric force in the area of the workpiece, in a manner generally similar to that shown in FIGURE 8, such lines of electric force, however, being more concentrated in the central area occupied by the workpiece 13.

The alternatives of providing either an absorber or a reflector at the open end or ends of the waveguide, as discussed in detail in relation to FIGURES 1 to 6, are equally applicable to the circularly polarized embodiments of FIGURES 7 and 8.

What is claimed is:

1. Apparatus for subjecting an elongated tubular cylindrical workpiece to dielectric heating, comprising (a) a substantially electrically continuous waveguide of cross-section having substantial orthogonal symmetry and defining a central longitudinal axis,

(b) means for moving said workpiece along its own longitudinal axis along said central waveguide axis,

(c) and means for propagating microwave energy along said waveguide in either direction of said waveguide axis at a wavelength at which at least a part of said workpiece will absorb said energy to generate heat and principally in the TE mode yielding a concentration of lines of electric force 5 6 in the portion of the waveguide occupied by the (h) and means for injecting said energy into said first workpiece. rectangular waveguide in 'a manner to propagate 2. Apparatus according to claim 1, wherein the wavetherein in the TE mode and be transmitted to the guide is of circular cross-section. waveguide containing the workpiece and injected 3. Apparatus according to claim 1, wherein said propa- 5 thereinto for propagation therealong in the TE gating means comprises means for injecting said energy mode.

into the waveguide at a location intermediate the ends thereof to travel in both longitudinal directions away References cued from said intermediate location. UNITED STATES PATENTS 4 Apparatus according to claim 1, wherein said propa- 10 2 40 142 5 5 Kim X 8 2 3 i l zllf f g t d f n d th 3,307,010 2/1967 Piischner 219--l0.55

our s ots 1str1= u e clrcurn erentra y 'aroun e wall of the waveguide at locations displaced 90 FOREIGN PATENTS from each other, 1,452,124 8/1966 France. (e) a first rectangular waveguide bifiurcated to form a 15 1,092,484- 11/1967 Great Britain.

second and a third rectangular waveguide, (f) said second rectangular waveguide being bifurcated OTHER REFERENCES to form a i f waveguide arms, h h arm Radar Electromcs Fundamentals, Bureau of Shlps, Navy having a wall opening communicating with one of a Dfiptq 111116 1944 PP- 364 to air of adjacent said slots, 0 (g? said third rectangular waveguide being bifurcated 2 JOSEPH TRUHE Pnmary Exammer to form a further pair of waveguide arms, each arm L H BENDER, A i t E i of said further pair having a wall opening communicating with one of the other pair of adjacent said US. Cl. X.R. slots, 25 219-1061 

